Device for compensation of an alternating voltage which occurs between a medium and a metallic pipeline disposed in the medium

ABSTRACT

In a metallic pipeline (1) disposed in the ground, alternating voltages may be induced from adjacent transmission lines and cause corrosion. To reduce the risk of corrosion, an alternating current (i 1 ) is caused by means of a compensating device (PD) to flow through the line and to cause a voltage drop which counteracts the induced voltages. A measuring conductor (2) provides a signal (u s ) which is a measure of the induced voltage. This signal controls the amplitude and phase position of an alternating voltage (u 1 ) which is generated by an a.c. source which is connected to feed points (A, B) on the line and which causes an alternating current (i 1 )) to flow through the line. (FIG. 1a)

TECHNICAL FIELD

The invention relates to a device for compensation of an alternatingvoltage which occurs between a medium and a metallic pipeline disposedin the medium, the pipeline being surrounded by a layer (mantle) ofelectrically insulating material.

BACKGROUND ART

In case of parallelism between a.c. transmission lines and metal pipesfor, for example, natural gas, the normal operating current of thetransmission line induces a voltage in the metal pipe. For example, froma 400 kV line with an operating current of 1000 A at a distance of 50 mfrom the pipeline, an induced voltage of about 20 V/km can be obtained.

A metal pipe of the above kind may, for example, constitute part of along gas conduit, which is disposed in the ground and possibly partiallyalso in water. A conduit of this kind is usually divided into sectionswith the aid of electrically insulating joints. The length of onesection may vary from several kilometers up to several tens ofkilometers. If a transmission line runs parallel to such a line for adistance of some length, induced voltages of a considerable magnitudemay therefore occur.

When the alternating voltage between the pipe and the surrounding ground(water) exceeds a few tens of volts, this may entail an increased riskof corrosion damage to the pipeline because of electrolytic corrosion.Metal pipes of the kind in question are provided with a protectivecoating of an electrically insulating material. However, damageunavoidably arises in this coating, whereby the metal pipe is broughtinto electrical contact with the surrounding medium. At these points theabove-mentioned risk of corrosion occurs.

Different types of measures for protection against corrosion arepreviously known. However, these do not provide any protection againstthe risk of corrosion which is caused by alternating voltages induced ina pipeline.

A previously know device for corrosion protection of a pipeline disposedin the ground or in water is disclosed in Swedish published patentapplication 466 160. This device aims at protecting the pipeline againstcorrosion caused by potential differences in the medium surrounding thepipeline (ground potential differences). The device primarily relates tothose cases where the ground potential differences are caused byelectric currents flowing in the medium, which typically originate fromelectric d.c. power plants. The difference in ground potential betweentwo points on the pipeline is sensed by means of ground electrodesdisposed near the line. A d.c. source is connected to two points on theline and is adapted to feed a current through the line between thesepoints. The current is controlled in such a way in dependence on thesensed ground potential difference that the voltage drop caused by thecurrent along the pipe corresponds to the ground potential difference.

It is also stated that the device can be designed so as to protectagainst corrosion in those cases where the ground currents consist ofalternating currents. The ground potential difference then consists ofan alternating voltage, and instead of a d.c. source an a.c. source isarranged to drive a suitable alternating current through the pipeline.

This known device provides corrosion protection for those cases wherethe risk of corrosion originates from ground potential differences.However, the device provides no protection at all against the risk ofcorrosion which is caused by voltages induced in the pipeline.

SUMMARY OF THE INVENTION

The invention aims to provide a device which, in a simple andadvantageous manner, provides good protection against the risks ofcorrosion which, in pipelines of the kind mentioned in the introduction,are caused by alternating voltages induced in the pipelines.

What characterizes a device according to the invention will become clearfrom the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail with reference to theaccompanying FIGS. 1a, 1b, 2a, 2b, 3, 4, 5a and 5b, wherein

FIG. 1a shows an example of a device according to the invention,

FIG. 1b shows the reduction of the voltage between the pipeline and thesurrounding medium which is obtained with the aid of the deviceaccording to the invention shown in FIG. 1a,

FIG. 2a shows how, according to the invention, several compensatingdevices and supply sections can be arranged along a section of thepipeline,

FIG. 2b illustrates function of the equipment shown in FIG. 2a,

FIG. 3 shows an alternative embodiment in which the current to a supplysection is obtained with the aid of a power amplifier,

FIG. 4 shows how a controllable transformer connection can be used as analternative for generating the current to a supply section, and

FIGS. 5a and 5b show an alternative method for controlling the currentto the supply section of the pipeline.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1a shows an elementary diagram of a piece of equipment PD accordingto the invention. The figure shows a section 1 of a metallic natural gasconduit 1 disposed in the ground, the conduit being provided with anelectrically insulating coating and being electrically insulated fromadjoining pipe sections with the aid of electrically insulating joints11 and 12. To provide a measure of the alternating voltage which can beinduced in the section 1 by electric transmission lines, which extend inthe vicinity of and wholly or partially parallel to the pipe section, ameasuring conductor 2 insulated from ground is arranged. This conductormay be arranged in the ground, on the ground or above the ground. Themeasuring conductor 2 is suitably arranged parallel to the pipeline andclose to this. The length of the measuring conductor may be small inrelation to the length of the section 1, but if desirable for obtaininga sufficient magnitude of the measured signal from the conductor, or formaking the measured signal sufficiently representative for the voltageinduced in the line section, the length of the conductor may constitutea considerable part of the length of the section. The measuringconductor may be arranged along all of or part of the supply section A-Bof the pipeline, or, as shown in the figure, it may be displaced inrelation thereto. The conductor 2 is insulated from ground but maypossibly be grounded at a suitable point. The voltage u_(s) induced inthe conductor 2 is supplied to an instrument amplifier 3, the outputsignal of which is designated u'_(s). Due to the location of themeasuring conductor 2 parallel to and close to the pipe section 1, thesignals u_(s) and u'_(s) become a good measure of the voltage in thepipe section induced by the operating current of the transmission line.The signal u'_(s) from the instrument amplifier 3 is supplied to anabsolute value generator 4 and a phase detector 5. The absolute valuegenerator 4 delivers a signal U which is proportional to the amplitudeof the voltage u_(s) induced in the measuring conductor 2. The phasedetector 5 delivers a signal φ which is proportional to the phasedifference between the signal u'_(s) and a reference voltage u_(ref).The reference signal is an alternating signal with the same frequency asthe frequency in the transmission line which causes the voltages inducedin the pipeline. As shown in the figure, the reference voltage can beobtained in the simplest manner from a local network 6, which belongs tothe same power network as the above-mentioned transmission line andtherefore has the same frequency as this.

The signals U and φ are supplied to an a.c. source 7, which is connectedto two feed points A and B in the pipeline. That part of the pipeline,the supply section, which is located between points A and B is suppliedfrom the a.c. source (which has the output voltage u₁) with analternating current i₁, which in the supply section generates a voltagedrop du_(AB). The amplitude of the voltage drop becomes proportional tothe amplitude of the current i₁, and the proportionality constantconsists of the absolute value of the impedance of the supply section.The impedance of a typical pipeline is about 0.3 ohm/km, and thisimpedance is practically purely inductive--the resistance is negligiblecompared with the inductance.

As shown in FIG. 1a, the a.c. source 7 may consist of an alternatingvoltage converter, for example an intermediate link converter with acontrollable rectifier supplied from the network 6, a direct voltageintermediate link, and a self-commutated inverter adapted to deliver analternating voltage with a controllable frequency and hence with acontrollable phase position. In this embodiment of the controller 7, thevoltage U is adapted to control the intermediate link direct voltage andhence the amplitude of the voltage u₁ delivered by the converter, andthe signal φ is adapted to control the inverter such that the voltage u₁is given the desired phase position in relation to the referencevoltage. Since the impedance of the supply section is nearly purelyinductive, the voltage drop du_(AB) across the supply section has aphase lead of almost 90 degrees in relation to the current i₁. Theamplitude and phase position of the voltage u₁ are controlled such thatthis voltage drop is substantially in opposition to the voltage inducedin the line section and is given a suitable magnitude in relation to theinduced voltage. In this connection, the impedances of the lines betweenthe current source 7 and the connection points A and B must be taken inaccount. Since both these impedances and the impedance of the supplysection may be assumed to be nearly constant, this fact can be takeninto account in a simple manner when controlling the current source 7.Thus, the current source can suitably be controlled such that theamplitude of the voltage u₁ becomes a first constant times the sensedvoltage amplitude u_(s), and such that the phase position of the voltageu₁ becomes the sensed phase position plus a second constant.

With a correct choice of the two constants mentioned above, as is clearfrom the above-mentioned voltage drop, du_(AB) will lie in opposition tothe EMF induced in the pipeline by the transmission line. These two EMFswill thus counteract each other, and with a correct design andadjustment of the equipment according to the invention, a goodcompensation may be obtained of the voltages induced in the pipeline 1by the transmission line current, that is, a considerable reduction ofthe maximum voltage between the pipeline and ground, and hence aconsiderable reduction or a complete elimination of the risk ofcorrosion. The two constants mentioned above are chosen and adjustedinto the control system such that the desired degree of suppression isobtained of the voltage induced in the pipeline. The constants can bedetermined by calculation, measurement or by practical tests.

If considered necessary, the signal from the measuring conductor 2 canbe filtered in a band-pass filter tuned to the frequency of thetransmission line, the reason being to eliminate the effect of voltagesoccurring in the measuring conductor and originating from sources otherthan the transmission line.

FIG. 1b shows the voltage u_(rg) in the pipeline in relation to groundplotted against the distance x from one end of the line section. Thesection is assumed to have the length 1 and be grounded at its centre,for example through damage to the electrical insulation of the line. Thecurve designated a in the figure shows the voltage between the pipelineand ground which would be caused by a transmission line extending inparallel with the line section along the whole of its length. Thevoltage assumes a maximum value ±u_(m) at the end points of the section.If a supply section A-B according to the invention is arranged at thecentral part of the line section and adapted to generate in the pipelinea voltage drop du_(AB), the voltage will have an appearance as shown bythe curve b, where u_(AB) is the voltage induced in the pipeline betweenpoints A and B. As will be clear from the figure, a considerablereduction of the maximum voltage between the pipeline and ground isobtained according to the invention.

In the example shown in FIG. 1b, the current i₁ to the supply sectionA-B, and hence the voltage drop du_(AB) across the supply section, areso chosen in dependence on the induced voltage that the maximum voltagebetween the pipeline and ground (which occurs at points A and B and atthe ends of the section) becomes as low as possible. In dependence onthe individual circumstances, of course, the voltage drop across thesupply section can be set in a different way.

A further reduction of the maximum voltage between the pipeline andground can be obtained according to an embodiment of the invention inwhich several supply sections with associated current sources arearranged along a line section. FIG. 2a shows part of such a linesection. A first compensating device PD1 is connected to the connectionpoints A and B. It has a measuring conductor 21 and delivers, independence on the voltage induced in the measuring conductor, a currenti₁ to the supply section. In a corresponding way, a second compensatingdevice PD2 is connected to the connection points C and D and has ameasuring conductor 22, the induced voltage of which controls thecurrent i₂ of the device. Further, a third compensating device PD3 isconnected to the feed points E and F and has a measuring conductor 23,the voltage of which controls the current i₃ of the device. In FIG. 2b,the curve b shows the voltage which is obtained between the pipeline andground. As is shown, a more complete reduction of the maximum voltagebetween the line and ground can be obtained in this way. In FIG. 2b, forthe sake of simplicity the voltage drops du_(AB), du_(CD), du_(EF)generated by the three compensating devices are shown equally great. Inpractice, however, the signals of the measuring conductor will differfrom each other--and hence also the voltage drops across the threesupply sections--and this in such a way as to obtain the best possiblecompensation of the induced voltage.

Possibly, the three compensating devices in FIG. 2a may be controlledfrom one single common measuring conductor. Possibly, also the controlequipment (units 3, 4, 5 in FIG. 1a) may then be common to the threedevices which then only have separate output stages (corresponding tounit 7 in FIG. 1a). Possibly, the output stage may also be common, inwhich case, however, this must be connected to the supply sections viatransformers to obtain the necessary galvanic separation.

Also in the case shown in FIGS. 1a and 2a with one separate compensatingdevice per supply section, it may be advantageous to connect thecompensating device to the supply section via a transformer in order toadapt the a.c. source, with respect to current and voltage levels, toits load--the supply section.

FIG. 3 shows an alternative embodiment of the equipment according to theinvention. The signal u'_(s) from the instrument amplifier 3 is suppliedto a power amplifier 9 which supplies to the supply section A-B acurrent i₁ proportional to the measured signal. By a suitable design ofthe circuit, the voltage drop across the supply section will be inopposition to the voltage induced in the line section, and by a suitableadjustment of the amplification factor of the amplifier, in principle acomplete suppression of the voltages induced in the pipeline 1 can beobtained, independently of their frequencies. The amplifier 9 may, forexample, be a switched power amplifier of a kind known per se.

FIG. 4 shows how, as an alternative, a transformer connection can beused for generating the supply voltage to a supply section. Theconnection comprises two single-phase transformers 22 and 23. Thetransformer 22 has its primary winding connected to the phases S and Tof the local network 6, and the transformer 23 has its primary windingconnected between the phase R and the neutral line 0 of the network. Theamplitude of the output voltage of each transformer is controllable,continuously or in steps. The transformers may, for example, consist ofservo-motor operated adjustable transformers or of transformers whichare provided with tap changers. In the connection shown, the outputvoltage U_(A) from the transformer 23 will have a phase shift of 90° inrelation to the output voltage U_(B) from the transformer 22.

Since the secondary windings of the two transformers are connected inseries, their output voltages will be added vectorially, and theirvector sum constitutes the supply voltage u₁ to the supply section. Ifthe output voltage of each transformer can be varied from maximumamplitude in one phase position to maximum amplitude in the oppositephase position, the output voltage u₁ may in a known manner becontrolled arbitrarily both with respect to amplitude and phase positionwithin all four quadrants. For control of the transformers, the signalsU and φ (see FIG. 1a) are supplied to a control unit 21, which deliverscontrol signals s1 and s2 to the actuators of the transformers. Thecontrol device may, for example, deliver such control signals s1 and s2to the transformers that the output voltages thereof become:

    U.sub.A =k.sub.1 ·U·sin (φ+k.sub.2)

    U.sub.B =k.sub.1 ·U·sin (φ+k.sub.2)

By a suitable choice of the constants k₁ and k₂, the supply voltage u₁to the supply section of the pipeline section can be given such anamplitude and such a phase position that the voltage drop in the supplysection compensates for the induced voltage.

The control of the equipment according to the invention can be carriedout in other ways than the one described above. For example, as shown inFIG. 5a, the induced voltage may be sensed at a plurality of locationsdistributed along the pipeline. In the example of FIG. 5a, this is doneby means of a plurality of measuring conductors 21, 22, 23, the inducedvoltages of which are supplied to instrument amplifiers 31, 32, 33. Theoutput signals u'_(s1), u'_(s2), u'_(s3) of the instrument amplifiersare supplied to an optimization unit 34 (FIG. 5b). This, in turn,delivers a control signal s3 to the controller 7. The control signal s3influences the amplitude and phase position of the voltage u₁ generatedby the controller and hence of the current i₁ which is supplied to thesupply section. The optimization unit 34 may, for example, consist of asuitably programmed computer adapted to influence the current i₁ via thecontrol signal s3 in such a way in dependence on the measured signalsthat the risk of corrosion of the pipeline is minimized.

The measuring conductors 2 described above constitute one way of forminga measure of the voltage induced in the pipeline. Also other ways arefeasible. The voltage induced in the pipeline is, of course, inmagnitude and in phase position, directly dependent on the load currentof the power transmission. In those cases where it is possible andsuitable to measure this current, it can be used directly as a measureof the voltage induced in the pipeline.

In the above description, it has been implicitly assumed that the loadcurrent in the transmission line, and hence the voltage induced in thepipeline, is a pure sine wave without harmonics. In practice, harmonicsmay occur in the load current and induce alternating voltages ofcorresponding frequencies in the pipeline, which voltages, in the sameway as the fundamental component, may cause risks of corrosion. Theembodiment of the invention shown in FIG. 3 will automatically entail acompensation also of induced harmonics, since the current i₁ applied tothe supply section constitutes a sign-reversed reproduction of themeasured signal u_(s) obtained from the measuring conductor 2. Harmonicsin the induced voltage may, of course, be compensated for also in otherways. Thus, for example, both the fundamental component and theharmonics in question may be separated out of the measured signal withthe aid of band-pass filters and be determined individually in amplitudeand phase position, whereupon the desired voltage u₁ and/or current i₁for suppressing all the sensed components are synthetized in a suitableway with the aid of suitable electronic circuits.

As an alternative to the converter connection 7 shown in FIG. 1 and tothe transformer connection shown in FIG. 4, a cascade connection of aninduction regulator and an adjustable transformer can be used, theinduction regulator being used for controlling the phase position of thesupply voltage to the supply section and the adjustable transformerbeing used for controlling the amplitude of the voltage.

In the embodiments of the invention described above, the output voltage(u_(s)) of the measuring conductor controls the voltage (u₁) of the a.c.source with the aid of a non-feedback control system. Alternatively, afeedback control system may be used, in which case, for example, thecurrent (i₁) delivered by the a.c. source is sensed and compared inamplitude and phase position with the measured signal or with areference quantity formed from the measured signal.

We claim:
 1. A device for compensation of an alternating voltage inducedin a metallic, electrically insulated pipeline and directed in thelongitudinal direction of the pipeline, said pipeline is disposed in amedium, wherein the device comprises:(a) first members forming a firstquantity which corresponds to an alternating voltage induced in thelongitudinal direction of the pipeline, and (b) a controllable a.c.source connected to connection points on the pipeline, said connectionpoints spaced from each other in the longitudinal direction of thepipeline, said a.c. source supplying said first quantity and, independence thereon, to cause an alternating current to flow in thelongitudinal direction of the pipeline with such an amplitude and such aphase position that the voltage drop in the longitudinal direction ofthe pipeline tends to reduce the voltage difference between the pipelineand the medium.
 2. A device according to claim 1, wherein the firstmembers comprise a measuring conductor disposed in the medium adjacentthe pipeline and insulated from the medium, and the first quantity isformed from an alternating voltage induced in the measuring conductor.3. A device according to claim 1, wherein the first members compriseamplitude-sensing members forming an amplitude signal corresponding tothe amplitude of the induced voltage, and phase-angle sensing membersforming a phase position signal corresponding to the phase position ofthe induced voltage, which signals are supplied to the a.c. source,which in turn is supplied to the pipeline as a current with an amplitudecorresponding to the amplitude signal and a phase position correspondingto the phase position signal.
 4. A device according to claim 3, whereinthe phase-angle sensing members form the phase position signal independence on the phase position of the in induced voltage in relationto a reference alternating voltage, and the a.c. source generates analternating current with the same frequency as the reference alternatingvoltage and with a phase position, in relation to the referencealternating voltage, dependent on the phase position signal.
 5. A deviceaccording to claim 1, wherein the pipeline has a plurality of pairs ofconnection points and an a.c. source connects each pair of connectionpoints.
 6. A device according to claim 1, wherein the a.c. sourceconsists of a converter connection.
 7. A device according to claim 1,wherein the a.c. source consists of a power amplifier.
 8. A deviceaccording to claim 1, wherein the a.c. source consists of a controllabletransformer connection.